Familiar elements of photonic and phononic systems such as waveguides, splitters, resonant cavities, and frequency filters are realized in materials in which photonic or phononic bandgaps exist or can be fabricated. Waveguides in photonic crystalline arrays (photonic crystals) have been proposed for many photonic applications, but photonic crystals have the disadvantage that they are highly anisotropic, placing tight constraints on the bending angles for waveguides and prohibiting any waveguide sections that do not align with the high symmetry directions in the crystal. Cavities in photonic crystals have also been proposed and fabricated for use in many photonic applications but, again, confinement of the radiation is anisotropic, making the properties of the cavities more difficult to control and introducing losses (leakage of the radiation). The photonic environment offered by crystalline material provides the strongest confinement along the high-symmetry directions of the crystal (along which are placed scattering centers), but is less effective along the other directions.
Phononic crystals are analogous to photonic crystals in that they are fashioned from inhomogeneous materials characterized by periodic variations in their mechanical properties (Sigalas, M., and Economou, E. J. Sound Vib, 158: 377-382, 1992). The electrical and magnetic waves that traverse photonic crystals (except in photonic bandgaps) are analogous, in some ways (Estrada, H. et al., Phys. Rev. Lett. 102: 144301, 2009) to the acoustic and elastic waves that traverse phononic materials (except in phononic bandgaps)